Viruses similar to hepatitis B virus ("hepadnaviruses") have been identified in three non-human species: the eastern woodchuck, the Beechey ground squirrel and the Pekin duck. It is likely that many other species harbor similar viruses. The existing animal hosts are of limited value because inbred strains suitable for detailed immunological studies are not available. We are searching for hepadnaviruses among inbred strains of rodents, especially those with a known high incidence of hepatoma, in hopes of finding a more useful animal model system. Sensitive assays of hepadnavirus infection have been modified to permit testing of the small quantities of serum available.